U.S. patent application number 12/426150 was filed with the patent office on 2009-10-29 for stainless separator for fuel cell and method of manufacturing the same.
This patent application is currently assigned to HYUNDAI HYSCO. Invention is credited to Kyeong Woo Chung, Yoo Taek JEON.
Application Number | 20090269649 12/426150 |
Document ID | / |
Family ID | 40931432 |
Filed Date | 2009-10-29 |
United States Patent
Application |
20090269649 |
Kind Code |
A1 |
JEON; Yoo Taek ; et
al. |
October 29, 2009 |
STAINLESS SEPARATOR FOR FUEL CELL AND METHOD OF MANUFACTURING THE
SAME
Abstract
A stainless steel separator for fuel cells and a method of
manufacturing the same are disclosed. The method includes preparing
a stainless steel sheet as a matrix, performing surface
modification on a surface of the stainless steel sheet to form a
Cr-rich passive film having a comparatively increased amount of Cr
in a superficial layer of the stainless steel sheet by decreasing
an amount of Fe in the superficial layer of the stainless steel
sheet, and forming a coating layer on the surface of the
surface-modified stainless steel sheet. The coating layer is one
selected from a metal nitride layer (MN.sub.x), a metal/metal
nitride layer (M/MN.sub.x), a metal carbide layer (MC.sub.y), and a
metal boride layer (MB.sub.z) (where 0.5.ltoreq.x.ltoreq.1,
0.42.ltoreq.y.ltoreq.1, 0.5.ltoreq.z.ltoreq.2).
Inventors: |
JEON; Yoo Taek; (Yongin-shi,
KR) ; Chung; Kyeong Woo; (Seoul, KR) |
Correspondence
Address: |
AMPACC LAW GROUP
3500 188th St. SW
Lynnwood
WA
98037
US
|
Assignee: |
HYUNDAI HYSCO
Ulsan
KR
|
Family ID: |
40931432 |
Appl. No.: |
12/426150 |
Filed: |
April 17, 2009 |
Current U.S.
Class: |
429/434 ;
427/115 |
Current CPC
Class: |
C22C 38/008 20130101;
Y02E 60/50 20130101; H01M 2008/1095 20130101; C23C 22/46 20130101;
C22C 38/44 20130101; C23C 22/73 20130101; C23G 1/088 20130101; C23C
22/76 20130101; C23C 14/0036 20130101; H01M 8/0228 20130101; Y02P
70/50 20151101; C23C 8/26 20130101; H01M 50/403 20210101; C21D
6/004 20130101; C22C 38/42 20130101; C23G 1/085 20130101; C21D
9/0068 20130101; C23C 8/04 20130101; C23C 22/82 20130101; C23C 8/08
20130101; C23C 14/48 20130101; C25D 7/06 20130101; Y02E 60/10
20130101; C23C 22/50 20130101; C25D 5/50 20130101; C25D 11/34
20130101; H01M 8/021 20130101; C23C 8/22 20130101 |
Class at
Publication: |
429/34 ;
427/115 |
International
Class: |
H01M 2/16 20060101
H01M002/16; H01M 2/00 20060101 H01M002/00; H01M 8/02 20060101
H01M008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 23, 2008 |
KR |
10-2008-0037916 |
May 6, 2008 |
KR |
10-2008-0041799 |
Claims
1. A method of manufacturing a stainless steel separator for fuel
cells, comprising: preparing a stainless steel sheet as a matrix;
performing surface modification on a surface of the stainless steel
sheet to form a Cr-rich passive film having a comparatively
increased amount of Cr in a superficial layer of the stainless
steel sheet by decreasing an amount of Fe in the superficial layer
of the stainless steel sheet; and forming a coating layer on the
surface of the surface-modified stainless steel sheet, the coating
layer being one selected from a metal nitride layer (MN.sub.x), a
metal/metal nitride layer (M/MN.sub.x), a metal carbide layer
(MC.sub.y), and a metal boride layer (MB.sub.z) (where
0.5.ltoreq.x.ltoreq.1, 0.42.ltoreq.y.ltoreq.1,
0.5.ltoreq.z.ltoreq.2).
2. The method according to claim 1, wherein the metal (M) used in
the forming a coating layer is at least one selected from chromium
(Cr), titanium (Ti), zirconium (Zr), and tungsten (W).
3. The method according to claim 1, wherein the metal (M) used in
the forming a coating layer is selected from transition metals.
4. The method according to claim 1, wherein the Cr-rich passive
film constituting the superficial layer of the stainless steel
sheet has a (Cr+Ni)/Fe ratio of 1 or more in terms of atomic weight
ratio.
5. The method according to claim 1, wherein the coating layer is
formed in a film shape having a thickness of 30.about.300 nm.
6. The method according to claim 1, wherein the surface
modification comprises immersing the stainless steel sheet in a
solution comprising sulfuric acid (H.sub.2SO.sub.4) and nitric acid
(HNO.sub.3), or spraying the solution onto the surface of the
stainless steel sheet.
7. The method according to claim 1, wherein the surface
modification comprises immersing the stainless steel sheet in a
surface modification solution comprising sulfuric acid
(H.sub.2SO.sub.4), and applying a potential (or current) in an SHE
region of greater than 0 to 1.0V.
8. The method according to claim 6, wherein the surface
modification solution further comprises one or more additives
selected from hydrogen peroxide (H.sub.2O.sub.2) and oxalic acid
(C.sub.2H.sub.2O.sub.4).
9. The method according to claim 1, wherein the coating layer is
formed by sputtering or arc ion plating.
10. The method according to claim 9, wherein the sputtering is
reactive sputtering.
11. The method according to claim 1, wherein the stainless steel
sheet contains 16.about.28 wt % chromium.
12. A method of manufacturing a stainless steel separator for fuel
cells, comprising: preparing a stainless steel sheet as a matrix;
performing surface modification on a surface of the stainless steel
sheet to form a Cr-rich passive film having a comparatively
increased amount of Cr in a superficial layer of the stainless
steel sheet by decreasing an amount of Fe in the superficial layer
of the stainless steel sheet; and heat-treating the
surface-modified stainless steel sheet at 100.about.300.degree. C.
under vacuum, in air or in an inert gas atmosphere.
13. The method according to claim 12, wherein the heat treatment is
performed at a temperature of 100.about.200.degree. C.
14. The method according to claim 12, wherein the inert gas is at
least one selected from nitrogen gas (N.sub.2), argon gas (Ar),
helium gas (He), and hydrogen gas (H.sub.2).
15. The method according to claim 12, wherein the Cr-rich passive
film constituting the superficial layer of the stainless steel
sheet has a (Cr+Ni)/Fe ratio of 1 or more in terms of atomic weight
ratio.
16. The method according to claim 12, wherein the heat treatment is
performed for 3 minutes to 1 hour.
17. The method according to claim 12, wherein the surface
modification comprises immersing the stainless steel sheet in a
solution comprising sulfuric acid (H.sub.2SO.sub.4) and nitric acid
(HNO.sub.3), or spraying the solution onto the surface of the
stainless steel sheet.
18. The method according to claim 12, wherein the surface
modification comprises immersing the stainless steel sheet in a
surface modification solution comprising sulfuric acid
(H.sub.2SO.sub.4), and applying a potential (or current) in an SHE
region of greater than 0 to 1.0V.
19. The method according to claim 17, wherein the surface
modification solution further comprises one or more additives
selected from hydrogen peroxide (H.sub.2O.sub.2) and oxalic acid
(C.sub.2H.sub.2O.sub.4).
20. The method according to claim 12, wherein the heat treatment is
selectively performed by one of batch type heat treatment and
continuous line heat treatment.
21. The method according to claim 12, wherein the stainless steel
sheet contains 16.about.28 wt % chromium.
22. A stainless steel separator for fuel cells, comprising: a
stainless steel sheet as a matrix; a Cr-rich passive film formed on
a surface of the stainless steel sheet and containing 20.about.75
wt % chromium; and a coating layer formed on the Cr-rich passive
film and having a thickness of 30.about.300 nm, the coating layer
being one selected from a metal nitride layer (MN.sub.x), a
metal/metal nitride layer (M/MN.sub.x), a metal carbide layer
(MC.sub.y), and a metal boride layer (MB.sub.z)
(0.5.ltoreq.x.ltoreq.1, 0.42.ltoreq.y.ltoreq.1,
0.5.ltoreq.z.ltoreq.2).
23. The stainless steel separator according to claim 22, wherein
the coating layer is formed in a continuous film shape on the
passive film.
24. The stainless steel separator according to claim 22, wherein
the metal (M) applied to the coating layer is at least one selected
from chromium (Cr), titanium (Ti), zirconium (Zr), and tungsten
(W).
25. The stainless steel separator according to claim 22, wherein
the passive film has a (Cr+Ni)/Fe ratio of 1 or more in terms of
atomic weight ratio.
26. The stainless steel separator according to claim 22, wherein
the stainless steel sheet comprises 0.08 wt % or less carbon (C),
16.about.28 wt % chromium (Cr), 0.1.about.20 wt % nickel (Ni),
0.1.about.6 wt % molybdenum (Mo), 0.1.about.5 wt % tungsten (W),
0.1.about.2 wt % tin (Sn), 0.1.about.2 wt % copper (Cu), and the
balance of iron (Fe) and unavoidable impurities.
27. The stainless steel separator according to claim 22, wherein
the stainless steel separator has a corrosion current density less
than 1 .mu.A/cm.sup.2 and a contact resistance less than 20
m.OMEGA.cm.sup.2 on both surfaces thereof.
28. A stainless steel separator for fuel cells, comprising: a
stainless steel sheet as a matrix; and a Cr-rich passive film
formed on a surface of the stainless steel sheet and containing
20.about.75 wt % chromium, wherein the stainless steel separator is
heat treated at 100.about.300.degree. C. under vacuum, in air or in
an inert gas atmosphere.
29. The stainless steel separator according to claim 28, wherein
the passive film has a (Cr+Ni)/Fe ratio of 1 or more in terms of
atomic weight ratio.
30. The stainless steel separator according to claim 28, wherein
the stainless steel separator has a corrosion current density less
than 1 .mu.A/cm.sup.2 and a contact resistance less than 20
m.OMEGA.cm.sup.2 on both surfaces thereof.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a stainless steel separator
for fuel cells and a method of manufacturing the same. More
particularly, the present invention relates to a stainless steel
separator for fuel cells and a method of manufacturing the same,
which is used for polymer electrolyte fuel cells (PEMFCs) and
exhibits superior corrosion resistance, electrical conductivity,
and durability.
[0003] 2. Description of the Related Art
[0004] Since a unit cell of a fuel cell stack generates too low a
voltage to be used alone in practice, the fuel cell stack generally
includes several to several hundred unit cells stacked therein.
When stacking the unit cells, a separator or bipolar plate is used
to facilitate electrical connection between the unit cells while
separating a reaction gas.
[0005] The bipolar plate is an essential component of the fuel cell
along with a membrane electrode assembly (MEA) and has a variety of
functions, such as structural support for the MEA and gas diffusion
layers (GDLs), collection and transfer of current, transmission and
removal of reaction gas, transmission of water coolant for removing
reaction heat, etc.
[0006] Hence, it is necessary for materials of the bipolar plate to
have excellent electrical and thermal conductivity, air-tightness,
chemical stability, and the like.
[0007] Graphite-based materials and composite graphite materials
composed of resin and graphite are employed as the materials for
the bipolar plate.
[0008] However, the graphite-based material has lower strength and
air-tightness than metallic materials, and demands high
manufacturing costs irrespective of low productivity when applied
to the bipolar plate. Recently, metallic bipolar plates have been
actively investigated to overcome such problems of the graphite
bipolar plate.
[0009] When the bipolar plate is made of a metallic material, there
are many merits in that volume and weight reduction of a fuel cell
stack can be accomplished via thickness reduction of the bipolar
plate, and in that the bipolar plate can be fabricated by stamping
and the like, thereby ensuring mass production of the bipolar
plates.
[0010] However, the metallic material inevitably undergoes
corrosion during use of the fuel cell, causing contamination of the
MEA and performance deterioration of the fuel cell stack. Further,
a thick oxide film can be grown on the metal surface after extended
use of the fuel cell, thereby causing an increase in internal
resistance of the fuel cell.
[0011] Stainless steel, titanium alloys, aluminum alloys, nickel
alloys, and the like are proposed as candidate materials for the
bipolar plate of the fuel cell. Particularly, stainless steel has
received attention due to its low price and good corrosion
resistance, but further improvements in corrosion resistance and
electrical conductivity are still needed.
SUMMARY OF THE INVENTION
[0012] The present invention is conceived to solve the problems of
the related art as described above, and an aspect of the present
invention is to provide a method of manufacturing a stainless steel
separator for fuel cells that has corrosion resistance and contact
resistance satisfying the standards of the Department of Energy
(DOE) not only at an initial stage but also after exposure to high
temperature-high humidity conditions in the fuel cell for a long
duration.
[0013] Another aspect of the present invention is to provide a
stainless steel separator manufactured by the method.
[0014] According to one embodiment of the present invention, a
method of manufacturing a stainless steel separator for fuel cells
includes: preparing a stainless steel sheet as a matrix; performing
surface modification on a surface of the stainless steel sheet to
form a Cr-rich passive film having a comparatively increased amount
of Cr in a superficial layer of the stainless steel sheet by
decreasing an amount of Fe in the superficial layer of the
stainless steel sheet; and forming a coating layer on the surface
of the surface-modified stainless steel sheet, the coating layer
being one selected from a metal nitride layer (MN.sub.x), a
metal/metal nitride layer (M/MN.sub.x), a metal carbide layer
(MC.sub.y), and a metal boride layer (MB.sub.z) (where
0.5.ltoreq.x.ltoreq.1, 0.42.ltoreq.y.ltoreq.1,
0.5.ltoreq.z.ltoreq.2).
[0015] According to another embodiment of the present invention, a
method of manufacturing a stainless steel separator for fuel cells
includes: preparing a stainless steel sheet as a matrix; performing
surface modification on a surface of the stainless steel sheet to
form a Cr-rich passive film having a comparatively increased amount
of Cr in a superficial layer of the stainless steel sheet by
decreasing an amount of Fe in the superficial layer of the
stainless steel sheet; and heat-treating the surface-modified
stainless steel sheet at 100.about.300.degree. C. under vacuum, in
air or in an inert gas atmosphere.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The above and other aspects, features and advantages of the
present invention will become apparent from the following
description of exemplary embodiments given in conjunction with the
accompanying drawings, in which:
[0017] FIG. 1 is a flowchart of a method of manufacturing a
stainless steel separator according to one embodiment of the
present invention;
[0018] FIGS. 2 to 4 are perspective views illustrating the
respective steps of the method shown in FIG. 1;
[0019] FIG. 5 is a cross-sectional view of a contact resistance
tester for measuring contact resistance of a stainless steel sheet
according to the present invention;
[0020] FIG. 6 is a graph depicting results of evaluating corrosion
resistance of Example 1 and Comparative Examples 2 and 3 in a
simulated fuel cell environment;
[0021] FIG. 7 is a graph depicting results of evaluating contact
resistance of Examples 1, 4, 8 to 18 and Comparative Examples 1 and
2 in a simulated fuel cell environment;
[0022] FIG. 8 is a graph depicting results of evaluating long-term
durability of Examples 1, 4, 8 to 18 and Comparative Example 1 in a
simulated fuel cell environment;
[0023] FIG. 9 is a flowchart of a method of manufacturing a
stainless steel separator according to another embodiment of the
present invention;
[0024] FIG. 10 is a graph depicting results of evaluating contact
resistance of Examples 19, 21, 23 and 26, and Comparative Examples
4, 6 to 8 in a simulated fuel cell environment;
[0025] FIG. 11 is a graph depicting results of evaluating corrosion
current density of Examples 19 and 21, and Comparative Example 4
exposed to a simulated fuel cell environment for 2,000 hours;
and
[0026] FIG. 12 is a graph depicting results of evaluating long-term
durability of Examples 19, 21, 23 and 26, and Comparative Example
4, 6 to 8 in a simulated fuel cell environment.
DETAILED DESCRIPTION OF THE EMBODIMENT
[0027] Hereinafter, exemplary embodiments of the present invention
will be described in detail with reference to the accompanying
drawings.
[0028] However, it should be noted that the present invention is
not limited to the embodiments and can be realized in various
forms, and that the following embodiments are given by way of
illustration to provide a thorough understanding of the invention
to those skilled in the art. Therefore, the present invention is
defined only by the accompanying claims. Like elements will be
denoted by like reference numerals throughout the
specification.
[0029] Further, it should be noted that the drawings are not to
precise scale and may be exaggerated in thickness of layers, films
and/or regions for descriptive convenience and clarity only. When a
certain film or layer is described as being formed "on" another
film or layer, the certain film or layer may be disposed directly
on the other film or layer, or may be disposed above the other film
or layer with a third film or layer interposed therebetween.
[0030] FIG. 1 is a flowchart of a method of manufacturing a
stainless steel separator according to one embodiment of the
present invention, and FIGS. 2 to 5 are perspective views
illustrating the respective steps of the method shown in FIG.
1.
[0031] To manufacture a stainless steel separator according to the
embodiment of the invention, a stainless steel sheet 200 is
prepared in S110, as shown in FIG. 2.
[0032] In this embodiment, the stainless steel sheet 200 is a
stainless steel sheet which is readily available in the marketplace
and contains 16.about.28 wt % chromium. Alternatively, the
stainless steel sheet may contain about 18 wt % chromium.
[0033] Specifically, a matrix of the stainless steel sheet 200 is a
stainless steel sheet that comprises 0.08 wt % or less carbon (C),
16.about.28 wt % chromium (Cr), 0.1.about.20 wt % nickel (Ni),
0.1.about.6 wt % molybdenum (Mo), 0.1.about.5 wt % tungsten (W),
0.1.about.2 wt % tin (Sn), 0.1.about.2 wt % copper (Cu), and the
balance of iron (Fe) and unavoidable impurities. More specifically,
the stainless steel sheet is an austenite stainless steel such as
SUS 316L 0.2t.
[0034] This operation may include a cleaning process for removing
impurities from the surface of the stainless steel sheet 200 using
acid and alkali degreasers before performing subsequent surface
modification and formation of a coating layer.
[0035] Next, as shown in FIG. 3, the surface of the stainless steel
sheet 200 is subjected to surface modification in S120.
[0036] Although the stainless steel sheet 200 contains chromium and
nickel components exhibiting high corrosion resistance, the
stainless steel sheet 200 is mainly composed of iron (Fe).
[0037] As a result, in a natural state, the stainless steel sheet
200 tends to react with oxygen in air to form an oxide film on the
surface of the stainless steel sheet. Here, since the oxide film is
an insulator, it can cause deterioration of the overall electrical
conductivity of the stainless steel sheet 200.
[0038] Therefore, there is a need for surface modification on the
surface of the stainless steel sheet which undergoes deterioration
in corrosion resistance.
[0039] In other words, the surface modification is performed for
selectively etching only the iron component (Fe) in a superficial
layer of the stainless steel sheet 200.
[0040] After the surface modification, the surface of the stainless
steel sheet 200 becomes a Cr-rich passive film 210. The Cr-rich
passive film 210 contains 20.about.75 wt % chromium and 30 wt % or
less iron, and has a (Cr+Ni)/Fe ratio of 1 or more as expressed by
a ratio of main components in the Cr-rich passive film 210.
[0041] Here, the selective metal dissolution can be accomplished
because iron oxide in the superficial oxide film can be easily
dissolved in an acid whereas chromium oxide therein is more stable
than the iron oxide and does not easily dissolve in acids.
[0042] Next, a solution and conditions for the surface modification
will be described.
[0043] A surface modification solution comprises 5.about.20 wt %
pure nitric acid (HNO.sub.3), 2.about.15 wt % pure sulfuric acid
(H.sub.2SO.sub.4), and the balance of water. The surface
modification may be performed at 50.about.80.degree. C. for an
immersion duration of 30 seconds to 30 minutes or less. Here, the
surface modification may be performed for 30 seconds to 10 minutes
or less while adjusting the concentrations of the nitric acid and
the sulfuric acid in consideration of productivity according to
treatment duration.
[0044] According to one embodiment of this invention, the surface
modification solution may be prepared by adding one or both of
oxalic acid (C.sub.2H.sub.2O.sub.4) and hydrogen peroxide
(H.sub.2O.sub.2) to the aforementioned surface modification
solution (nitric acid+sulfuric acid) to accelerate a metal
dissolution rate on the surface of the stainless steel sheet.
[0045] Further, for the surface modification, an electrochemical
process may be carried out by applying an SHE potential of greater
than 0.0 to 1.0 V to the stainless steel sheet which has been
immersed in the surface modification solution comprising sulfuric
acid (H.sub.2SO.sub.4), thereby enabling selective dissolution of
Fe in a further reduced period of time.
[0046] With the surface modification, a large amount of Fe and a
part of Ni content are selectively dissolved to reduce the amount
of Fe in the superficial layer of the stainless steel sheet without
substantially dissolving chromium (Cr) therein, so that the
chromium and nickel components are concentrated on the superficial
layer of the stainless steel sheet.
[0047] After the surface modification, the Cr-rich passive film 210
may have a thickness of 5.about.100 nm.
[0048] Next, a coating layer is formed on the Cr-rich passive film
210 in S130, as shown in FIGS. 4 and 5.
[0049] The coating layer 220 may be selected from a metal nitride
layer (MN.sub.x), a metal/metal nitride layer (M/MN.sub.x), a metal
carbide layer (MC.sub.y), and a metal boride layer (MB.sub.z). The
formation of the coating layer is performed for the following
reasons.
[0050] When the stainless steel sheet 200 is modified, the Cr-rich
passive film 210 is formed on the stainless steel sheet as
described above, thereby ensuring superior corrosion resistance and
electrical conductivity at an initial stage.
[0051] However, when a surface-modified stainless steel separator
is exposed for long durations to high temperature-high humidity
conditions of a fuel cell, the passive film is gradually thickened.
Here, since the passive film mainly consists of metallic oxides,
the stainless steel separator undergoes deterioration in electrical
conductivity after a predetermined operating period even though the
corrosion resistance is maintained.
[0052] Accordingly, the coating layer 220 selected from the metal
nitride layer (MN.sub.x), the metal/metal nitride layer
(M/MN.sub.x), the metal carbide layer (MC.sub.y) and the metal
boride layer (MB.sub.z) and having both superior corrosion
resistance and superior electrical conductivity is formed on the
Cr-rich passive film 210, so that the separator for the fuel cell
can be prepared to have superior corrosion resistance and superior
electrical conductivity not only at an initial operating stage but
also after long-term operation.
[0053] Here, metal (M) constituting the coating layer 220, which is
selected from the metal nitride layer (MN.sub.x), the metal/metal
nitride layer (M/MN.sub.x), the metal carbide layer (MC.sub.y) and
the metal boride layer (MB.sub.z), may be selected from transition
metals, which have both superior corrosion resistance and superior
electrical conductivity in nitride form. Specifically, the metal
may selected from chromium (Cr), titanium (Ti), zirconium (Zr), and
tungsten (W) (where 0.5.ltoreq.x.ltoreq.1, 0.42.ltoreq.y.ltoreq.1,
0.5.ltoreq.z.ltoreq.2).
[0054] The coating layer 220 may have a thickness of 30.about.300
nm, and preferably a thickness of 30.about.100 nm. The coating
layer having a thickness less than 30 nm provides insignificant
effects, whereas the coating layer having a thickness greater than
300 nm deteriorates productivity due to a high price of a metal
target and a long-term process.
[0055] The coating layer 220 selected from the metal nitride layer
(MN.sub.x), the metal/metal nitride layer (M/MN.sub.x), the metal
carbide layer (MC.sub.y) and the metal boride layer (MB.sub.z) may
be obtained, without limitation, by arc ion plating or physical
vapor deposition such as sputtering and the like.
[0056] In this embodiment, since reactive sputtering permits easy
control of the process, the reactive sputtering is used for forming
the coating layer 220 selected from the metal nitride layer
(MN.sub.x), the metal/metal nitride layer (M/MN.sub.x), the metal
carbide layer (MC.sub.y) and the metal boride layer (MB.sub.z).
[0057] Examples of the metal (M) constituting the coating layer 220
selected from the metal nitride layer (MN.sub.x), the metal/metal
nitride layer (M/MN.sub.x), the metal carbide layer (MC.sub.y) and
the metal boride layer (MB.sub.z) include chromium (Cr), titanium
(Ti), zirconium (Zr), and tungsten (W).
[0058] Although sputtering is used in this embodiment, other
processes may also be used to form the coating layer 220.
[0059] To form the coating layer 220, a metal target having a
purity of 99.99% or more may be used as a sputtering target.
[0060] A technique for forming the coating layer by sputtering will
be described in more detail. After the stainless steel sheet 200
and the metal target are loaded in a sputtering chamber, sputtering
is performed in an atmosphere of argon and nitrogen (Ar+N.sub.2)
gas to form the coating layer 220 on the passive film 210 of the
stainless steel sheet.
[0061] When forming a coating layer consisting of two layers of
M/MN.sub.x, argon gas is supplied alone to form a metal layer (M),
followed by supplying the argon and nitrogen (Ar+N2) gas to form a
MN.sub.x layer continuous with the metal layer (M), so that the two
layer of M/MN.sub.x can be continuously formed.
[0062] As such, in this process, the sputtering is performed in an
argon gas atmosphere when forming the metal layer (M), and is then
performed in the atmosphere of argon and nitrogen (Ar+N.sub.2) gas
when forming the metal nitride layer (MN.sub.x).
[0063] In more detail, referring to FIG. 4, the coating layer 220
is formed in a continuous film shape on the passive film 210.
EXAMPLES AND COMPARATIVE EXAMPLES
[0064] Next, a description of the present invention will be given
with reference to inventive and comparative examples to show that a
stainless steel separator for fuel cells manufactured by the method
according to the embodiment of this invention has excellent
corrosion resistance and contact resistance. A description of
details apparent to those skilled in the art will be omitted
herein.
TABLE-US-00001 TABLE 1 CrN coating Pickling Coating Corrosion Temp.
Time layer Thickness rate CR Process (.degree. C.) (min)
Composition design (nm) (.mu.A/cm.sup.2) (m.OMEGA.cm.sup.2) E1 IM
60 3 15% HNO.sub.3 + 10% CrN 30 0.75 14.6 H.sub.2SO.sub.4 E2 IM 60
3 5% HNO.sub.3 + 5% CrN 50 0.72 15.1 H.sub.2SO.sub.4 + 5% Oxalic E3
IM 60 3 5% HNO.sub.3 + 5% CrN 50 0.73 14.9 H.sub.2SO.sub.4 + 5%
H.sub.2O.sub.2 E4 IM 60 3 10% HNO.sub.3 + 5% Cr/CrN 300 0.58 14.3
H.sub.2SO.sub.4 + 5% H.sub.2O.sub.2 multilayer E5 EC 60 100 1M
H.sub.2SO.sub.4 CrN 50 0.76 14.6 E6 EC 60 100 1M H.sub.2SO.sub.4
Cr/CrN 100 0.79 15.2 multilayer E7 EC 60 100 1M H.sub.2SO.sub.4 CrN
100 0.82 14.3 E8 EC 60 100 1M H.sub.2SO.sub.4 TiN 100 0.74 14.9 E =
Example, IM = Immersion process, EC = Electrochemical process, CR:
Contact resistance
[0065] Tables 1, 2 and 3 show corrosion currents and contact
resistances of stainless steel separators of Examples 1 to 18 and
Comparative Examples prepared using stainless steel 316 L as
matrices of the stainless steel separators via the immersion
process and the electrochemical process under different conditions
for surface modification (temperature, time, current density, and
solution composition) and under different conditions (kind, design,
and thickness of coating layer) in formation of a coating
layer.
TABLE-US-00002 TABLE 2 CrN coating Pickling Coating Corrosion Temp.
Time layer Thickness rate CR Process (.degree. C.) (min)
Composition design (nm) (.mu.A/cm.sup.2) (m.OMEGA.cm.sup.2) E9 EC
60 100 1M H.sub.2SO.sub.4 ZrN 100 0.81 13.8 E10 IM 60 3 15%
HNO.sub.3 + 10% Cr.sub.2N 100 0.89 13.9 H.sub.2SO.sub.4 E11 IM 60 3
15% HNO.sub.3 + 10% TiC 100 0.85 16.1 H.sub.2SO.sub.4 E12 IM 60 3
15% HNO.sub.3 + 10% ZrC 100 0.81 16.6 H.sub.2SO.sub.4 E13 IM 60 3
15% HNO.sub.3 + 10% Cr.sub.3C.sub.2 100 0.83 16.9 H.sub.2SO.sub.4
E14 IM 60 3 15% HNO.sub.3 + 10% Cr.sub.7C.sub.3 100 0.84 16.6
H.sub.2SO.sub.4 E15 IM 60 3 15% HNO.sub.3 + 10% CrB.sub.2 100 0.92
15.9 H.sub.2SO.sub.4 E16 IM 60 3 15% HNO.sub.3 + 10% TiB.sub.2 100
0.85 16.1 H.sub.2SO.sub.4 E = Example, IM = Immersion process, EC =
Electrochemical process, CR: Contact resistance
[0066] Specifically, Examples 1 to 7, and 10 were subjected to both
surface modification and formation of a chromium nitride layer (CrN
or Cr.sub.2N) (Examples 4 and 6 had Cr/CrN multiple layers).
Examples 8, 9 and 11 to 18 were formed with coating layers of
titanium nitride, titanium carbide and titanium boride (TiN, TiC
and TiB.sub.2), zirconium nitride, zirconium carbide and zirconium
boride (ZrN, ZrC and ZrB.sub.2), chromium carbide and chromium
boride (Cr.sub.3C.sub.2, Cr.sub.7C.sub.3 or CrB.sub.2), and
tungsten carbide (WC), respectively. Comparative Example 1 was
formed with a coating layer of chromium nitride (CrN) having a
thickness of 15 nm which is greater than the thickness of the
coating layer 220 according to the present invention. Comparative
Example 2 was subjected only to surface modification without the
formation of the coating layer, and Comparative Example 3 was
formed with only a chromium nitride (CrN) layer without the surface
modification.
TABLE-US-00003 TABLE 3 Pickling CrN coating Corrosion Temp. Time
Coating Thickness rate CR Process (.degree. C.) (min) Composition
layer design (nm) (.mu.A/cm.sup.2) (m.OMEGA.cm.sup.2) E17 IM 60 3
15% HNO.sub.3 + 10% ZrB.sub.2 100 0.81 16.6 H.sub.2SO.sub.4 E18 IM
60 3 15% HNO.sub.3 + 10% WC 100 0.86 17.6 H.sub.2SO.sub.4 CE1 IM 60
3 15% HNO.sub.3 + 10% CrN 15 0.94 17.3 H.sub.2SO.sub.4 CE2 EC 60
100 1M H.sub.2SO.sub.4 -- -- 0.95 17.5 CE3 -- -- -- -- CrN 30 2.3
35 E = Example, IM = Immersion process, EC = Electrochemical
process, CR: Contact resistance
[0067] 1. Measurement of Contact Resistance
[0068] FIG. 5 is a cross-sectional view of a contact resistance
tester for measuring contact resistance of a stainless steel
separator according to one embodiment of the present invention.
[0069] Referring to FIG. 5, in order to obtain optimized parameters
for cell assembly for measurement of contact resistance of a
stainless steel sheet 500, a modified Davies method was used to
measure contact resistance between stainless steel (SS) and two
pieces of carbon paper.
[0070] The contact resistance was measured based on the principle
of measuring four-wire current-voltage via a contact resistance
tester available from Model IM6 from Zahner Inc.
[0071] Measurement of the contact resistance was performed by
application of DC 2 A and AC 0.2 A to a measuring target in a
constant current mode at a frequency in the range of 10 kHz to 10
mHz.
[0072] The carbon paper was 10 BB available from SGL Inc.
[0073] In the contact resistance tester 50, a sample 500 was
disposed between two pieces of carbon paper 520 and copper plates
510 connected to both a current supplier 530 and a voltage tester
540.
[0074] Voltage was measured by applying DC 2 A/AC 0.2 A to the
sample 500 using a current supplier 530 (Model IM6 from Zahner
Inc.).
[0075] Then, the sample 500, carbon paper 520, and copper plates
510 were compressed to form a stacked structure from both copper
plates 510 of the contact resistance tester 50 using a pressure
regulator (Model No. 5566 from Instron Inc., compression
maintaining test). Using the pressure regulator, a pressure of
50.about.150 N/cm.sup.2 was applied to the contact resistance
tester 50.
[0076] The contact resistances of samples 500, that is, stainless
steel sheets, of the inventive and comparative examples shown in
Tables 1 and 2 were measured using the contact resistance tester 50
prepared as described above.
[0077] 2. Measurement of Corrosion Current Density
[0078] A corrosion current density of the stainless steel sheet
according to the present invention was measured using EG&G
Model No. 273A as a corrosion current tester. Tests for corrosion
durability were performed in a simulated environment of a polymer
electrolyte fuel cell (PEFC).
[0079] After being etched at 80.degree. C. using 0.1N
H.sub.2SO.sub.4+2 ppm HF as an etching solution, the samples were
subjected to O.sub.2 bubbles for 1 hour, and the corrosion current
density thereof was measured at an open circuit potential (OCP) of
-0.25V.about.1V vs. SCE.
[0080] Other properties were measured at -0.24V vs. SCE for a PEFC
anode environment and at 0.6V vs. SCE for a PEFC cathode
environment.
[0081] Here, the measured properties were evaluated based on data
of corrosion current at 0.6V vs. SCE in a simulated cathode
environment of a fuel cell.
[0082] The anode environment is an environment in which hydrogen is
split into hydrogen ions and electrons while passing through a
membrane electrode assembly (MEA), and the cathode environment is
an environment in which oxygen combines with the hydrogen ions to
produce water while passing through the MEA.
[0083] Since the cathode environment has a high potential and is
very corrosive, it is desirable that the corrosion resistance be
tested in the cathode environment.
[0084] Further, it is desirable that the stainless steel sheet have
a corrosion current density of 1 .mu.A/cm.sup.2 or less for
application to the PEFC.
[0085] 3. Analysis of Measurement Results of Corrosion Current
Density and Contact Resistance
[0086] It can be seen from Tables 1 to 3 that, when the samples
were subjected to surface modification and were formed with metal
coating layers as in the inventive examples, the samples had a
corrosion current density of 0.5.about.1.0 .mu.A/cm.sup.2 and a
contact resistance of 13.about.18 m.OMEGA.cm.sup.2.
[0087] Comparative Example 1 having a 15 nm thick chromium nitride
layer formed after the surface modification had a contact
resistance of 17.4 m.OMEGA.cm.sup.2 and a corrosion current density
of 0.94 .mu.A/cm.sup.2. Comparative Example 2 subjected only to the
surface modification had a contact resistance of 17.5
m.OMEGA.cm.sup.2 and a corrosion current density of 0.95
.mu.A/cm.sup.2. Comparative Example 3 formed with a chromium
nitride layer without the surface modification had a contact
resistance of 35 m.OMEGA.cm.sup.2 and a corrosion current density
of 2.3 .mu.A/cm.sup.2.
[0088] Although the inventive examples and comparative examples had
contact resistances and corrosion current densities satisfying the
standards of the DOE, these values were merely initial values
before long-term operation of a fuel cell, and it can be seen that
a difference between the inventive examples and comparative
examples is significant in evaluation of long-term durability of
the fuel cell described below.
[0089] 4. Evaluation and Result of Corrosion Resistance and Contact
Resistance in Simulated Fuel Cell Environment
[0090] (1) Evaluation of Corrosion Resistance and Contact
Resistance in a Simulated Environment
[0091] For a simulated fuel cell environment to test a stainless
steel separator according to one embodiment of this invention,
EG&G Model No. 273A was used. After being immersed in 0.1N
H.sub.2SO.sub.4+2 ppm HF at 80.degree. C., samples were subjected
to O.sub.2 bubbles for 1 hour, followed by application of a
constant voltage of 0.6V vs. SCE. After applying the constant
voltage for a predetermined duration, the corrosion resistance and
contact resistance of each sample were measured. While repeating
this operation, the variation in corrosion resistance and contact
resistance in the simulated fuel cell environment over an extended
period of time was evaluated.
[0092] (2) Evaluation Results
[0093] FIG. 6 is a graph depicting results of evaluating the
corrosion resistance and the contact resistance in the simulated
fuel cell environment by the method as described above.
[0094] Referring to FIG. 6, Example 1 and Comparative Example 2 had
a corrosion current density of 1 .mu.A/cm.sup.2 not only at an
initial stage but also after 1,000 hours, whereas Comparative
Example 3 had a corrosion current density exceeding 1
.mu.A/cm.sup.2 not only at an initial stage but also after a long
time. It was considered that such a high corrosion current density
of Comparative Example 3 was caused by exfoliation of the CrN
layer.
[0095] FIG. 7 is a graph depicting results of evaluating the
contact resistance of Examples 1, 4, 8 to 18 and Comparative
Examples 1 and 2 exposed to a simulated fuel cell environment for
2,000 hours.
[0096] Referring to FIG. 7, Examples 1, 4, 8 to 18 had contact
resistances satisfying the standards of the DOE even after 2,000
hours, whereas Comparative Examples 1 and 2 had corrosion
resistances exceeding the standards of the DOE after 2,000
hours.
[0097] With regard to such high contact resistances of the
comparative examples, it was considered that Comparative Example 1
underwent exfoliation of the CrN layer or growth of the passive
film above the thickness of the CrN layer, and that Comparative
Example 2 underwent continuous growth of the passive film. On the
other hand, for the inventive examples, it was considered that a
metal compound layer formed on the surface of each separator
efficiently served as a corrosion resistant and electrically
conductive coating layer suppressing the growth of the passive film
under the metal compound layer.
[0098] 5. Evaluation and Result of Long-Term Durability of Fuel
Cell
[0099] (1) Evaluation of Long-Term Durability
[0100] Separators, each having a serpentine passage for supplying
reaction gas, were used. Each fuel cell was prepared by interposing
a membrane electrode assembly (Model 5710 from Gore Fuel Cell
Technologies, Inc.) and a gas diffusion layer (Model 10BA from SGL
Co., Ltd.) between the separators and compressing the same with a
predetermined pressure.
[0101] Performance of each of the fuel cells was evaluated using a
unit cell. NSE Test Station 700W class was used as a fuel cell
operator, and KIKUSUI E-Load was used as an electronic load for
evaluating the performance of the fuel cell. Current cycles of 0.01
A/cm.sup.2 current for 15 seconds and 1 A/cm.sup.2 current for 15
seconds were constantly applied.
[0102] As the reaction gas, hydrogen and air were supplied at a
flux with a stoichiometric ratio of H.sub.2 to air of 1.5:2.0
according to the electric current after being humidified to a
relative humidity of 100%. The performance of the fuel cell was
evaluated at atmospheric pressure while maintaining the temperature
of a humidifier and the cell at 65.degree. C. An active area was 25
cm.sup.2 and an operating pressure was 1 atm.
[0103] (2) Evaluation Results of Long-Term Durability
[0104] FIG. 8 is a graph depicting results of evaluating the
long-term durability of Examples 1, 4 and 8 to 18, and Comparative
Examples 1 and 2 by the method as described above.
[0105] Referring to FIG. 8, although all of the samples generated a
voltage of about 0.62 V or more at an initial stage, Comparative
Example 1 generated a decreased voltage of about 0.58 V after 2,000
hours and Comparative Example 2 generated a decreased voltage of
about 0.57 V after 2,000 hours.
[0106] Fuel cells including the stainless steel separators of
Examples 1, 4 and 8 to 18 experienced a minute reduction of voltage
less than 0.02 V even after 2,000 hours due to superior durability
of the stainless steel separators.
[0107] FIG. 9 is a flowchart of a method of manufacturing a
stainless steel separator according to another embodiment of the
present invention.
[0108] To manufacture a stainless steel separator according to this
embodiment of the invention, a stainless steel sheet is prepared in
S910.
[0109] In this embodiment, the stainless steel sheet is a stainless
steel sheet which is readily available in the marketplace and
contains 16.about.28 wt % chromium. Alternatively, the stainless
steel sheet may contain about 18 wt % chromium.
[0110] Specifically, a matrix of the stainless steel separator is a
stainless steel sheet that comprises 0.08 wt % or less carbon (C),
16.about.28 wt % chromium (Cr), 0.1.about.20 wt % nickel (Ni),
0.1.about.6 wt % molybdenum (Mo), 0.1.about.5 wt % tungsten (W),
0.1.about.2 wt % tin (Sn), 0.1.about.2 wt % copper (Cu), and the
balance of iron (Fe) and unavoidable impurities. More specifically,
the stainless steel sheet is an austenite stainless steel such as
SUS 316L 0.2t.
[0111] This operation may include a cleaning process for removing
impurities from the surface of the stainless steel sheet using acid
and alkali degreasers before performing subsequent surface
modification and formation of a coating layer.
[0112] Next, the surface of the stainless steel sheet is subjected
to surface modification in S920.
[0113] Although the stainless steel sheet contains chromium and
nickel components exhibiting high corrosion resistance, the
stainless steel sheet is mainly composed of iron (Fe).
[0114] As a result, in a natural state, the stainless steel sheet
tends to react with oxygen in air to form an oxide film on the
surface thereof. Here, since the oxide film is an insulator, it can
cause deterioration of the overall electrical conductivity of the
stainless steel sheet.
[0115] Therefore, there is a need for surface modification on the
surface of the stainless steel sheet which undergoes deterioration
in corrosion resistance.
[0116] In other words, the surface modification is performed for
selectively etching only the iron component (Fe) in a superficial
layer of the stainless steel sheet.
[0117] After the surface modification, the surface of the stainless
steel sheet becomes a Cr-rich passive film. The Cr-rich passive
film contains 20.about.75 wt % chromium and 30 wt % or less iron,
and has a (Cr+Ni)/Fe ratio of 1 or more as expressed by a ratio of
main components in the Cr-rich passive film.
[0118] Here, the selective metal dissolution can be accomplished
because iron oxide in the superficial oxide film can be easily
dissolved in an acid whereas chromium oxide therein is more stable
than the iron oxide and does not easily dissolve in acids.
[0119] Next, a solution and conditions for the surface modification
will be described.
[0120] A surface modification solution comprises 5.about.20 wt %
pure nitric acid (HNO.sub.3), 2.about.15 wt % pure sulfuric acid
(H.sub.2SO.sub.4), and the balance of water. The surface
modification may be performed at 50.about.80.degree. C. for an
immersion duration of 30 seconds.about.30 minutes or less. Here,
the surface modification may be performed for 30 seconds.about.10
minutes or less while adjusting the concentrations of the nitric
acid and the sulfuric acid in consideration of productivity
according to treatment duration.
[0121] According to one embodiment of this invention, the surface
modification solution may be prepared by adding one or both of
oxalic acid (C.sub.2H.sub.2O.sub.4) and hydrogen peroxide
(H.sub.2O.sub.2) to the aforementioned surface modification
solution (nitric acid+sulfuric acid) to accelerate a metal
dissolution rate on the surface of the stainless steel sheet.
[0122] Further, for the surface modification, an electrochemical
process may be carried out by applying an SHE potential of greater
than 0.0 to 1.0 V to the stainless steel sheet which has been
immersed in the surface modification solution comprising sulfuric
acid (H.sub.2SO.sub.4), thereby enabling selective dissolution of
Fe in a further reduced period of time.
[0123] With the surface modification, a large amount of Fe and a
part of Ni content are selectively dissolved to reduce the amount
of Fe in the superficial layer of the stainless steel sheet without
substantially dissolving chromium (Cr) therein, so that the
chromium and nickel components are concentrated on the superficial
layer of the stainless steel sheet.
[0124] After the surface modification, the Cr-rich passive film may
have a thickness of 5.about.100 nm.
[0125] Next, the stainless steel sheet subjected to the surface
modification and having the passive film on the surface thereof is
heat-treated in S930.
[0126] The heat treatment is performed for the following
reasons.
[0127] When the stainless steel sheet is subjected to the surface
modification, the Cr-rich passive film is formed on the surface of
the stainless steel sheet as described above, thereby ensuring
superior corrosion resistance and electrical conductivity at an
initial stage.
[0128] However, when the surface-modified stainless steel separator
is exposed for long durations to high temperature-high humidity
conditions of a fuel cell, the passive film is gradually thickened.
Since the passive film mainly consists of metallic oxides, the
stainless steel separator can suffer deterioration in electrical
conductivity after a predetermined operational period even though
the corrosion resistance thereof can be maintained.
[0129] Accordingly, the separator for the fuel cell can be prepared
to have superior corrosion resistance and electrical conductivity
not only at an initial operating stage of the fuel cell but also
after long-term operation thereof through heat treatment for
suppressing the growth of the passive film even after long-term
operation while ensuring both superior corrosion resistance and
electrical conductivity on the Cr-rich passive film.
[0130] The heat treatment may be performed under vacuum, in air, or
in an inert gas (for example, nitrogen, argon, helium, hydrogen,
etc.) atmosphere at a temperature of 100.about.300.degree. C., and
preferably at a temperature of 100.about.200.degree. C.
[0131] Heat treatment at a temperature of 100.degree. C. or less
provides an insignificant effect upon the stainless steel sheet. On
the other hand, if the heat treatment is performed above
300.degree. C., oxidation occurs on the surface of the stainless
steel sheet, deteriorating the properties thereof, and it is also
undesirable in view of manufacturing costs.
[0132] Although a heat treatment period is not specifically
limited, the heat treatment is advantageously performed for 3
minutes or more. Further, the heat treatment may be performed for 1
hour or less when taking into consideration a temperature
increasing time and costs.
[0133] In all examples of the present inventions described below,
the heat treatment was performed for 30 minutes.
[0134] The heat treatment may be performed in a batch-type manner
or a continuous line manner in a furnace.
[0135] The stainless steel separator for fuel cells manufactured by
the method according to this embodiment of the invention, that is,
through the surface modification and heat treatment, has a
corrosion current density of 1 .mu.A/cm.sup.2 or less and a contact
resistance of 20 m.OMEGA.cm.sup.2 or less on both surfaces, which
satisfy the standards of the DOE.
EXAMPLES AND COMPARATIVE EXAMPLES
TABLE-US-00004 [0136] TABLE 4 Surface modification Time (current
Heat treatment Corrosion Temp. density) Time Temp. rate CR Process
(.degree. C.) (min) Comp. Atm. (min) (.degree. C.) (.mu.A/cm.sup.2)
(m.OMEGA. cm.sup.2) E19 IM 60 3 15% HNO.sub.3 + Vacuum 30 200 0.54
12.2 10% H.sub.2SO.sub.4 (1 * 10.sup.-3 torr) E20 EC 60 (100) 1M
H.sub.2SO.sub.4 Vacuum 30 100 0.62 13.8 (1 * 10.sup.-3 torr) E21 EC
60 (100) 1M H.sub.2SO.sub.4 N.sub.2 30 200 0.61 12.8 E22 EC 60
(100) 1M H.sub.2SO.sub.4 N.sub.2 30 100 0.67 14.5 E23 EC 60 (100)
1M H.sub.2SO.sub.4 Ar 30 200 0.59 12.5 E24 EC 60 (100) 1M
H.sub.2SO.sub.4 Ar 30 100 0.64 14.1 E25 EC 60 (100) 1M
H.sub.2SO.sub.4 Air 30 100 0.57 17.5 E26 EC 60 (100) 1M
H.sub.2SO.sub.4 Air 30 200 0.49 17.6 E27 EC 60 (100) 1M
H.sub.2SO.sub.4 Air 30 300 0.43 17.8 CE4 EC 60 (100) 1M
H.sub.2SO.sub.4 -- -- -- 0.95 17.5 CE5 EC 60 (100) 1M
H.sub.2SO.sub.4 Air 30 400 0.35 23.3 CE6 EC 60 (100) 1M
H.sub.2SO.sub.4 Air 1 100 0.95 17.4 CE7 EC 60 (100) 1M
H.sub.2SO.sub.4 Air 1 200 0.94 17.4 CE8 EC 60 (100) 1M
H.sub.2SO.sub.4 Air 2 300 0.94 17.3 E = Example, CE = Comparative
Example, IM = immersion process, EC = electrochemical process,
Atm.: atmosphere, CR: Contact resistance
[0137] Table 4 shows corrosion currents and contact resistances of
stainless steel separators of Examples 19 to 27 and Comparative
Examples 4 to 8 prepared using stainless steel 316 L as matrices of
the stainless steel sheet separators under different conditions for
surface modification (temperature, time, and composition of
solution), heat treatment (atmosphere and temperature) by an
immersion process and an electrochemical process.
[0138] Specifically, Examples 19 and 20 were subjected to surface
modification and heat treatment in a vacuum of 1.times.10.sup.-3
torr. Examples 21 and 22 were subjected to surface modification and
heat treatment in a nitrogen atmosphere. Examples 23 and 24 were
subjected to surface modification and heat treatment in an argon
atmosphere as a Group 0 inert gas atmosphere. Examples 25 to 27
were subjected to surface modification and heat treatment in
air.
[0139] Comparative Example 4 was subjected to surface modification
without heat treatment, and Comparative Example 5 was subjected to
surface modification and heat treatment in air at a temperature of
400.degree. C. which did not satisfy the conditions of the present
invention. Comparative Examples 6 to 8 were subjected to surface
modification and heat treatment in air for 1 minute and 2 minutes,
respectively, which did not satisfy the conditions of the present
invention.
[0140] 1. Measurement of Contact Resistance
[0141] The contact resistances of samples 500, that is, stainless
steel sheets, of the inventive and comparative examples shown in
Table 4 were measured using the contact resistance tester 50 as
shown in FIG. 5.
[0142] 2. Measurement of Corrosion Current Density
[0143] The corrosion current density was measured by the same
method as in Examples 1 to 18.
[0144] 3. Analysis of Measurement Results of Corrosion Current
Density and Contact Resistance
[0145] Referring to Table 4, it can be understood that, when the
samples were subjected to the surface modification and heat
treatment satisfying the conditions of the present invention as in
the inventive examples, all of the samples had a corrosion current
density of 0.5.about.0.7 .mu.A/cm.sup.2 and a contact resistance of
12.about.18 m.OMEGA.cm.sup.2, all of which satisfy the standards
set by the DOE.
[0146] Comparative Example 4 subjected to the surface modification
without the heat treatment had a contact resistance of 17.5
m.OMEGA.cm.sup.2 and a corrosion current density of 0.95
.mu.A/cm.sup.2, and Comparative Example 5 subjected to the surface
modification and the heat treatment in air at a higher temperature
than that of the present invention had a contact resistance of 23.3
m.OMEGA.cm.sup.2 and a corrosion current density of 0.35
.mu.A/cm.sup.2. Comparative Examples 6 to 8 subjected to the
surface modification and the heat treatment in air for 1 minute had
a contact resistance of 17.3.about.17.4 m.OMEGA.cm.sup.2 and a
corrosion current density of 0.94.about.0.95 .mu.A/cm.sup.2.
[0147] Here, although Comparative Examples 4, 6 to 8 had the
contact resistance and the corrosion current satisfying the
standards of the DOE, these values were merely initial values
before long-term operation of a fuel cell, and it can be understood
that a difference between the inventive examples and Comparative
Example 4 is significant in evaluation of long-term durability of
the fuel cell described below.
[0148] 4. Evaluation and Results of Corrosion Resistance and
Contact Resistance in Simulated Fuel Cell Environment
[0149] (1) Evaluation of Corrosion Resistance and Contact
Resistance in a Simulated Fuel Cell Environment
[0150] The variation in corrosion resistance and contact resistance
in a simulated fuel cell environment was evaluated by the same
method as in Examples 1 to 18.
[0151] (2) Evaluation Results of Contact Resistance and Long-Term
Corrosion Resistance in a Simulated Fuel Cell Environment
[0152] FIG. 10 is a graph depicting evaluation results of the
contact resistance measured by the aforementioned method in the
simulated fuel cell environment.
[0153] Referring to FIG. 10, Examples 19, 21, 23 and 26 had a
contact resistance of 20 m.OMEGA.cm.sup.2 or less not only at an
initial stage (0 hour) but also after 2,000 hours, indicating that
the contact resistance thereof was substantially maintained.
Conversely, Comparative Examples 4, 6 to 8 had a contact resistance
of 17.3.about.17.5 m.OMEGA.cm.sup.2 at an initial stage as
described above, but had a contact resistance above 40
m.OMEGA.cm.sup.2 after 2,000 hours.
[0154] FIG. 11 is a graph depicting results of evaluating corrosion
current density of Examples 19 and 21 and Comparative Example 4
exposed to the simulated fuel cell environment for 2,000 hours as
described above.
[0155] Referring to FIG. 11, all of the samples of Examples 19 and
21 and Comparative Example 4 had corrosion current densities less
than or equal to the standards of the DOE not only at an initial
stage but also after 2,000 hours.
[0156] Therefore, it can be seen that, when the stainless steel
sheet was subjected to the surface modification without heat
treatment, the stainless steel separator could maintain corrosion
resistance but was significantly increased in surface resistance
after the long-term operation in the simulated fuel cell
environment.
[0157] 5. Evaluation and Results of Long-Term Durability of Fuel
Cell
[0158] (1) Evaluation Method of Long-Term Durability
[0159] The long-term durability was evaluated by the same method as
in Examples 1 to 18.
[0160] (2) Evaluation Results of Long-Term Durability
[0161] FIG. 12 is a graph depicting results of evaluating the
long-term durability of Examples 19, 21, 23 and 26 and Comparative
Examples 4, 6 to 8 by the method as described above.
[0162] Referring to FIG. 12, fuel cells of Comparative Examples 4,
and 6 to 8 generated a voltage of 0.6 V or more at an initial
stage, but generated a decreased voltage of about 0.57 V after
2,000 hours.
[0163] On the other hand, all of the fuel cells including the
stainless steel separators of Examples 19, 21, 23 and 26 generated
a voltage of 0.62 V or more at an initial stage, and experienced a
minute reduction of voltage less than 0.02 V even after 2,000 hours
due to superior durability of the stainless steel separators.
[0164] As apparent from the above description, the stainless steel
separator for fuel cells manufactured by the method according to an
embodiment of the invention has superior corrosion resistance and
electrical conductivity not only at an initial stage but also after
long-term use in operational conditions of the fuel cell.
[0165] Further, the method according to the embodiment of the
present invention enables surface modification for achieving
superior properties even with a general inexpensive stainless steel
sheet, thereby lowering manufacturing costs of the stainless steel
separator.
[0166] The stainless steel separator for fuel cells manufactured by
the method according to the embodiment of the invention has a
corrosion current density of 1 .mu.A/cm.sup.2 or less and a contact
resistance of 20 m.OMEGA.cm.sup.2 or less on both surfaces of the
separator.
[0167] Although some embodiments have been provided in conjunction
with the accompanying drawings to illustrate the present invention,
it will be apparent to those skilled in the art that the
embodiments are given by way of illustration, and that various
modifications and changes can be made without departing from the
spirit and scope of the present invention. Accordingly, the scope
of the present invention should be limited only by the accompanying
claims.
* * * * *